The primary purpose of a lubrication is separation of solid surfaces moving relative to one another, to minimise friction and wear. The materials most frequently used for this purpose are oils and greases. The choice of lubricant is mostly determined by the particular application.
Lubricating greases are employed where heavy pressures exist, where oil drip from the bearings is undesirable or where the motion of the contacting surfaces is discontinuous so that it is difficult to maintain a separating film in the bearing. Because of design simplicity, decreased sealing requirements and less need for maintenance, greases are almost universally given first consideration for lubricating ball and roller bearings in electric motors, household appliances, automotive wheel bearings, machine tools or aircraft accessories. Greases are also used for the lubrication of small gear drives and for many slow-speed sliding applications.
Lubricating greases consist primarily of a fluid lubricant, such as an oil, and a thickener. Essentially, the same type of oil is employed in compounding a grease as would normally be selected for oil lubrication. Fatty acid soaps of lithium, calcium, sodium, aluminium and barium are most commonly used as thickeners. However, thickeners may be one of a variety of solid materials, including clays, complexes such as those of lithium, and urea compounds.
The base oil may be of mineral or synthetic origin. Base oils of mineral origin may be mineral oils, for example produced by solvent refining or hydro-processing. Base oils of synthetic origin may typically be mixtures of C.sub.10-50 hydrocarbon polymers, for example liquid polymers of alpha-olefins. They may also be conventional esters, for example polyol esters. The base oil may also be a mixture of these oils. Preferably the base oil is that of mineral origin sold by the Royal Dutch/Shell Group of Companies under the designations "HVI" or "MVIN" or the synthetic hydrocarbon base oils sold by the Royal Dutch/Shell Group of Companies under the designation "XHVI" (trade mark).
The lubricating grease preferably contains 5 to 20% by weight of thickener.
Lithium soap thickened greases have been known for many years. Typically, the lithium soaps are derived from C.sub.10-24, preferably C.sub.15-18, saturated or unsaturated fatty acids or derivatives thereof. One particular derivative is hydrogenated castor oil, which is the glyceride of 12-hydroxystearic acid and is a particularly preferred fatty acid.
Greases thickened with complex thickeners are well known. In addition to a fatty acid salt, they incorporate into the thickener a complexing agent which is commonly a low to medium molecular weight acid or dibasic acid or one of its salts, such as benzoic acid or boric acid or a lithium borate.
Urea compounds used as thickeners in greases include the urea group (--NHCONH--) in their molecular structure. These compounds include mono-, di- or polyurea compounds, depending upon the number of urea linkages.
Various conventional grease additives may be incorporated into the lubricating greases, in amounts normally used in this field of application, to impart certain desirable characteristics to the grease, such as oxidation stability, tackiness, extreme pressure properties and corrosion inhibition. Suitable additives include one or more extreme pressure/antiwear agents, for example zinc salts such as zinc dialkyl or diaryl dithiophosphates, borates, substituted thiadiazoles, polymeric nitrogen/phosphorus compounds made, for example, by reacting a dialkoxy amine with a substituted organic phosphate, amine phosphates, sulfurized sperm oils of natural or synthetic origin, sulfurized lard, sulfurized esters, sulfurized fatty acid esters, and similar sulfurized materials, organo-phosphates for example according to the formula (OR).sub.3 P.dbd.O where R is an alkyl, aryl or aralkyl group, and triphenyl phosphorothionate; one or more overbased metal-containing detergents, such as calcium or magnesium alkyl salicylates or alkylarylsulphonates; one or more ashless dispersant additives, such as reaction products of polyisobutenyl succinic anhydride and an amine or ester; one or more antioxidants, such as hindered phenols or amines, for example phenyl alpha naphthylamine; one or more antirust or friction-modifying additives; one or more viscosity-index improving agents; one or more pour point depressing additives; and one or more tackiness agents. Solid materials such as graphite, finely divided molybdenum disulfide, talc, metal powders, and various polymers such as polyethylene wax may also be added to impart special properties.
In particular, the use of molybdenum disulfide is known from, for example, "Solid Lubricant Additives--Effect of Concentration and other Additives on Anti-Wear Performance", Bartz, Wear, 17 (1971) pages 421-432, to have the effect of reducing wear when incorporated in lubricating oils. Furthermore, in "Interrelations between Molybdenum Disulfide and Oil Soluble Additives", Bartz, NLGI Spokesman, December 1989, there is discussion of the use of molybdenum disulfide in combination with certain zinc dialkyldithio-phosphates. However, it is shown there that such a combination caused higher wear than when using either of those additives alone. Clearly such an antagonistic effect would make such a combination of additives quite unattractive for the reduction of friction levels.